Telescopic rods of the above mentioned type are known of many kinds. They differ from each other in the way for which they are arranged to increase or reduce their length and in the way for which they are locked/released at a desired length.
They can be used for handling a tool at a desired height above the head of a user, such as tools for harvesting fruit from trees, for cutting tree branches, for painting or brushing ceiling or side walls, for collecting objects from high shelves or closets, or for hanging objects at a desired height, etc.
The telescopic rods for these applications should be very light to permit a user to hold them easily and for a long working time, for example a whole day, and at the same time sufficiently stiff, to hold a tool which may be a motorised tool.
Powered telescopic rods are known for heavy duties, such as for raising loads, for moving working platforms or big tools, telescopic masts. Normally they are powered by hydraulic or pneumatic cylinders. However, hydraulic or pneumatic cylinders would be too bulky and heavy to power telescopic rods for the above applications.
Telescopic rods, also known for harvesting fruit or for cutting branches, have at an end a powered tool and a motor, as for example described in CN201878552. At the other end an electric power supply can be connected, for example a battery. The rod can be elongated or shortened manually and an extensible electrical connection is provided between the gripping end and the tool end, in order to supply the motor of the tool at any length of the rod.
Telescopic rods are also known, as for example described in KR20040072307, for harvesting fruit or for cutting branches that have at an end a tool. At the other end they have a transmission means for transmitting the power to the tool. The rod can be elongated or shortened manually, permitting to transmit the power to the tool at any desired length of the rod.
The steps of manually extending or shortening the telescopic rod require the user to lay the rod on a plane, for example on the ground, to unlock the sliding of the rod, to elongate/shorten the rod, to lock the sliding of the rod and then to raise the rod and operate the tool at the desired height.
However, it is desirable that the rod can be elongated/shortened when it is raised, in order to see directly when the tool is at the desired height from the ground and to avoid problematic steps of laying the rod on the ground, elongating/shortening it and then raising it again.
Telescopic rods are also known for example disclosed in U.S. Pat. No. 5,881,601 depicting an extensible rod which can carry out the extension retraction either manually or aided by a motor. The transmission between the fixed and movable part of the extensible rod comprises a rack. An intermediate member comprises gears that engage with the racks, permitting a relative movement of the two parts. The presence of racks and gears determines risk of jamming and blocking the relative movement, and therefore frequent maintenance.
U.S. Pat. No. 4,924,573 discloses an extensible rod with a screw operation. More precisely, a pruner is disclosed which has a fixed hollow base and a hollow rod extension slidingly engaging with the rod base. An actuation worm drive housed within the hollow base engages with a rack mounted to the rod extension. A motor transmission causes the worm to rotate, and a shift lever causes a gearbox to switch the rotation direction of the worm drive, in order to cause the rod extension to extend or to withdraw.
A disadvantage of the pruner disclosed in U.S. Pat. No. 4,924,573 is that, in order to increase the length of the rod extension, also the worm drive has to be correspondingly long. However, a long worm drive would vibrate transversally very intensely and would hit against the housing within the hollow base and also with the rod extension, disengaging from the rack. This would cause rapid wear and break of the worm drive. For this reason the disclosed pruner cannot be made with a long extension, unless reinforcing very heavily the cross section of the worm drive and of the other components, and then over sizing the whole rod and the weight of the parts and of all the device.
Another limit is that the worm drive rotation has to be significantly limited, to avoid the transversal vibration. However, this causes the extension/withdrawal speed to be very low, with a loss of most of the advantages of having a motorised telescopic rod.